Vaccines are considered to be one of the most successful and cost-effective medical interventions against infectious diseases (Hilleman M. R., Vaccine 18: 1436-1447, 2000). A vaccine is used to evoke an antigen-specific effectors and memory immune response against a pathogen, with minimal adverse reactions and it should lead to a specific long-term protection against this pathogen.
Traditional live anti-viral and anti-bacterial vaccines typically require no immunological adjuvants. Similarly, live microbial attenuated vaccines are generally much more immunogenic than killed pathogen or subunit protein vaccines and can be effective with no adjuvant or with adjuvants that have limited ability to stimulate immune responses. Recently developed killed pathogens, several vector types or subunit protein vaccines, while offering significant advantages over the traditional vaccines in terms of safety and cost of production, generally have limited immunogenicity compared to natural pathogens. As a result, these vaccines typically require adjuvants with significant immunostimulatory capability to reach their full potential in preventing disease.
A vaccine adjuvant is more precisely a particulate, solid or soluble agent that increases the specific immune responses to an antigen. Vaccine adjuvants can enhance the immune response to vaccine antigens in various ways. When weak antigens are available, they are very useful for augmenting the immunogenicity of these molecules, thereby enhancing their vaccinal potency. They are also used to enhance the speed, vigor, and persistence of the immune response to a strong antigen. They can also modify the nature of the immune response. Depending on which adjuvant is used to stimulate a protective immune response, humoral or cell-mediated immunity can be selected. An adjuvant can modulate antibody specificity, as well as its quantity, isotype and subclass distribution. When used in direct contact with mucous membrane (e.g., intranasal) it can effectively induce mucosal immunity. Adjuvants are also useful for potentiating the immune responses in immunologically immature, immunosuppressed or senescent individuals, acting as an immunological booster. Also, an adjuvant can effectively decrease the dose of antigen and/or the frequency of injection necessary to provide protection.
Adjuvants are immunomodulators that are typically non-covalently linked to antigens and are formulated to enhance the host immune responses. Some of these adjuvants are toxic, however, and can cause undesirable side effects, making them unsuitable for use in humans and many animals. Indeed, only few adjuvants are routinely used in human and veterinary vaccines. Also, currently available adjuvants and vaccines fail to induce a proper immune response capable of protecting against or treating certain infectious diseases, for instance those associated to Human immunodeficiency virus (HIV) or Hepatitis C virus (HCV).
Therefore, there is a need for the development of novel vaccine adjuvants and immunogenic compositions.
The present description refers to a number of documents, the content of which is herein incorporated by reference in their entirety.